Dual Margin Priority Circuit For Increased Steering Capacity

ABSTRACT

A dual pressure margin priority circuit and method for controlling flow from a pump to steering valve and low priority inlets. A steering pressure valve controls flow from pump to steering valve inlets, and provides a steering valve load sense pressure. A priority valve controls flow from pump to low priority inlets. A load sense cutoff valve has a first inlet receiving the steering valve load sense pressure. The load sense cutoff valve controls flow through the priority valve based on steering valve load sense pressure at the first cutoff valve inlet. The cutoff valve can include a second inlet coupled to tank, and a load sense input coupled to the steering valve load sense pressure. The cutoff valve can be a pressure limiter valve. The priority and steering pressure valves can be 2-way proportional flow spool valves with bias springs, and contributing and opposing load sense inputs.

FIELD OF THE DISCLOSURE

The present disclosure relates to hydraulic systems, and moreparticularly to a system and method to adjust hydraulic flow betweenhigher and lower priority hydraulic functions.

BACKGROUND

Increases in tractor size/tires/tracks/mass are all resulting in highersteering force requirements. Steering force is limited by the hydraulicpressure available to the steering circuit and/or the size of thesteering actuators. Increasing steering pressure is the most costeffective way to increase steering capacity as it requires minimalmodification to the steering valve and chassis components.Traditionally, a single spool/single pressure margin priority valve witha load sense relief feature is used to provide oil to a hydraulicsteering circuit. The load sense relief valve is used to prevent thepriority valve from blocking flow to low priority hydraulic functionswhen steering pressure reaches the relief setting. This approach reducesthe maximum pressure available at the steering valve inlet to less than90% of the maximum pump outlet pressure.

It would be desirable to enable the steering circuit to utilize themaximum pump pressure.

SUMMARY

A dual pressure margin priority circuit is disclosed that controls flowfrom a hydraulic pump to steering valve inlets and to low priorityfunction inlets. The dual pressure margin priority circuit includes asteering pressure compensation valve, a priority valve, a load sensecutoff valve and a load sense feed orifice. The steering pressurecompensation valve has an inlet port hydraulically coupled to the pumpand an outlet port hydraulically coupled to the steering valve inlets.The load sense feed orifice has an inlet port hydraulically coupled tothe outlet port of the steering pressure compensation valve to provide asteering valve dynamic load sense pressure. The load sense cutoff valvehas a first inlet port hydraulically coupled to the outlet port of theload sense feed orifice. The priority valve has an inlet porthydraulically coupled to the pump and an outlet port hydraulicallycoupled to the low priority function inlets. The load sense cutoff valvecontrols flow from the pump to the low priority function inlets throughthe priority valve based on the steering valve dynamic load sensepressure at the first inlet port of the load sense cutoff valve.

The pump can be a pressure compensated hydraulic pump that includes aload sense input, and the dual pressure margin priority circuit can alsoinclude a load sense resolution shuttle valve with a first inlet porthydraulically coupled to the steering valve dynamic load sense pressure,a second inlet port hydraulically coupled to a low priority functionload sense pressure and an outlet port hydraulically coupled to the loadsense input of the pump. The load sense resolution shuttle valve cancommunicate the greater of the steering valve dynamic load sensepressure at the first inlet port and the low priority function loadsense pressure at the second inlet port to the load sense input of thepump.

The load sense cutoff valve can also include a bias spring, a secondinlet port hydraulically coupled to a tank, and a load sense inputhydraulically coupled to the steering valve dynamic load sense pressureat the first inlet port of the load sense cutoff valve. When the forceof the bias spring is greater than the force of the steering valvedynamic load sense pressure at the load sense input of the load sensecutoff valve, the load sense cutoff valve can communicate the steeringvalve dynamic load sense pressure at the first inlet port to the outletport. When the force of the bias spring is less than the force of thesteering valve dynamic load sense pressure at the load sense input ofthe load sense cutoff valve, the load sense cutoff valve can connect theoutlet port to the tank. The force of the bias spring of the load sensecutoff valve can be adjustable.

The priority valve can be a 2-way proportional flow spool valve thatalso includes a bias spring, a contributing load sense inputhydraulically coupled to the outlet port of the load sense cutoff valve,and an opposing load sense input hydraulically coupled to the outletport of the steering pressure compensation valve. Pressure at thecontributing load sense input of the priority valve can aid the biasingforce of the bias spring to stop flow from the inlet port to the outletport of the priority valve, and pressure at the opposing load senseinput of the priority valve can oppose the biasing force of the biasspring to open flow from the inlet port to the outlet port of thepriority valve.

The steering pressure compensation valve can be a 2-way proportionalflow spool valve that also includes a bias spring, a contributing loadsense input hydraulically coupled to the outlet port of the load sensefeed orifice, and an opposing load sense input hydraulically coupled tothe outlet port of the steering pressure compensation valve. Pressure atthe contributing load sense input of the steering pressure compensationvalve can aid the biasing force of the bias spring to open flow from theinlet port to the outlet port of the steering pressure compensationvalve, and pressure at the opposing load sense input of the steeringpressure compensation valve can oppose the biasing force of the biasspring to stop flow from the inlet port to the outlet port of thesteering pressure compensation valve.

The dual pressure margin priority circuit can also include a load senseboost orifice with an inlet port hydraulically coupled to the outletport of the load sense feed orifice, and an outlet port hydraulicallycoupled to the first inlet port of the load sense cutoff valve and tothe first inlet port of the load sense resolution shuttle valve.Alternatively, the dual pressure margin priority circuit can include aload sense boost orifice with an inlet port hydraulically coupled to theoutlet port of the load sense feed orifice, and an outlet port of theload sense boost orifice hydraulically coupled to the first inlet portof the load sense resolution shuttle valve.

The load sense cutoff valve can be a pressure limiting valve that alsoincludes a bias spring, a load sense input hydraulically coupled to thesteering valve dynamic load sense pressure at the first inlet port ofthe load sense cutoff valve, and have the outlet port hydraulicallycoupled to a tank. When the force of the bias spring is greater than theforce of the steering valve dynamic load sense pressure at the loadsense input of the load sense cutoff valve, the load sense cutoff valvecan block flow from the inlet port to the outlet port of the load sensecutoff valve; and when the force of the bias spring is less than theforce of the steering valve dynamic load sense pressure at the loadsense input of the load sense cutoff valve, the load sense cutoff valvecan open flow from the inlet port to the outlet port of the load sensecutoff valve. The force of the bias spring of this type of pressurelimiting load sense cutoff valve can be adjustable.

With a pressure limiting load sense cutoff valve, the priority valve canbe a 2-way proportional flow spool valve that also includes a biasspring, a contributing load sense input hydraulically coupled to theinlet port of the load sense cutoff valve, and an opposing load senseinput hydraulically coupled to the outlet port of the steering pressurecompensation valve. Pressure at the contributing load sense input of thepriority valve can aid the biasing force of the bias spring to stop flowfrom the inlet port to the outlet port of the priority valve, andpressure at the opposing load sense input of the priority valve canoppose the biasing force of the bias spring to open flow from the inletport to the outlet port of the priority valve.

A method of controlling flow from a hydraulic pump to steering valveinlets and to low priority function inlets is disclosed that includescontrolling flow from the pump to the steering valve inlets using asteering valve; sensing a steering valve dynamic load sense pressure atan outlet port of the steering valve; controlling flow from the pump tothe low priority function inlets using a priority valve; controlling aload sense cutoff valve using the steering valve dynamic load sensepressure; and controlling flow from the pump to the low priorityfunction inlets through the priority valve using the load sense cutoffvalve based on the steering valve dynamic load sense pressure. Thepriority valve and the steering valve are separate valves.

Controlling a load sense cutoff valve using the steering valve dynamicload sense pressure can include hydraulically coupling the steeringvalve dynamic load sense pressure through the load sense cutoff valvefrom an inlet port of the load sense cutoff valve to a load sense inputof the priority valve when the steering valve dynamic load sensepressure is less than a threshold pressure; and hydraulically couplingthe load sense input of the priority valve through the load sense cutoffvalve from an outlet port of the load sense cutoff valve to a tank whenthe steering valve dynamic load sense pressure is greater than athreshold pressure. Controlling flow from the pump to the low priorityfunction inlets using a priority valve can include hydraulicallycoupling the outlet port of the load sense cutoff valve to acontributing load sense input of the priority valve; hydraulicallycoupling the outlet port of the steering valve to an opposing load senseinput of the priority valve; biasing the priority valve to block flowfrom the pump to the low priority function inlets using a priority biasspring force and the contributing load sense input of the priorityvalve; and biasing the priority valve to open flow from the pump to thelow priority function inlets using the opposing load sense input of thepriority valve. Controlling flow from the pump to the steering valveinlets using a steering valve can include hydraulically coupling theoutlet port of the steering valve to an inlet port of a load sense feedorifice; hydraulically coupling an outlet port of the load sense feedorifice to a contributing load sense input of the steering valve;hydraulically coupling the outlet port of the steering valve to anopposing load sense input of the steering valve; biasing the steeringvalve to open flow from the pump to the steering valve inlets using asteering bias spring force and the contributing load sense input of thesteering valve; and biasing the priority valve to block flow from thepump to the steering valve inlets using the opposing load sense input ofthe steering valve.

Controlling flow from the pump to the low priority function inlets usinga priority valve and controlling a load sense cutoff valve using thesteering valve dynamic load sense pressure can include hydraulicallycoupling the steering valve dynamic load sense pressure to an inlet portof the load sense cutoff valve and to a contributing load sense input ofthe priority valve; blocking the steering valve dynamic load sensepressure at the inlet port of the load sense cutoff valve when thesteering valve dynamic load sense pressure is less than a thresholdpressure; hydraulically coupling the inlet port of the load sense cutoffvalve and the contributing load sense input of the priority valvethrough the load sense cutoff valve to a tank when the steering valvedynamic load sense pressure is greater than a threshold pressure;hydraulically coupling the outlet port of the steering valve to anopposing load sense input of the priority valve; biasing the priorityvalve to block flow from the pump to the low priority function inletsusing a priority bias spring force and the contributing load sense inputof the priority valve; and biasing the priority valve to open flow fromthe pump to the low priority function inlets using the opposing loadsense input of the priority valve. Controlling flow from the pump to thesteering valve inlets using a steering valve can include hydraulicallycoupling the outlet port of the steering valve to an inlet port of aload sense feed orifice; hydraulically coupling an outlet port of theload sense feed orifice to a contributing load sense input of thesteering valve; hydraulically coupling the outlet port of the steeringvalve to an opposing load sense input of the steering valve; biasing thesteering valve to open flow from the pump to the steering valve inletsusing a steering bias spring force and the contributing load sense inputof the steering valve; and biasing the priority valve to block flow fromthe pump to the steering valve inlets using the opposing load senseinput of the steering valve.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned aspects of the present disclosure and the manner ofobtaining them will become more apparent and the disclosure itself willbe better understood by reference to the following description of theembodiments of the disclosure, taken in conjunction with theaccompanying drawings, wherein:

FIG. 1 illustrates an exemplary embodiment of a dual pressure marginpriority circuit with a load sense cutoff valve;

FIG. 2 illustrates another exemplary embodiment of a dual pressuremargin priority circuit with a load sense cutoff valve;

FIG. 3 illustrates an exemplary embodiment of a dual pressure marginpriority circuit with a pressure limiter valve; and

FIG. 4 illustrates another exemplary embodiment of a dual pressuremargin priority circuit with a pressure limiter valve.

Corresponding reference numerals are used to indicate correspondingparts throughout the several views.

DETAILED DESCRIPTION

The embodiments of the present disclosure described below are notintended to be exhaustive or to limit the disclosure to the preciseforms in the following detailed description. Rather, the embodiments arechosen and described so that others skilled in the art may appreciateand understand the principles and practices of the present disclosure.

A dual pressure margin (two spool) priority circuit can be used toenable the steering circuit to utilize greater pump pressure. This canresult in a steering capacity increase of over 10% without changing thesteering cylinders or the steering valve.

FIG. 1 illustrates an exemplary embodiment of a dual pressure marginpriority circuit 100 that can be used to enable the steering circuit toutilize greater pump pressure. The dual pressure margin priority circuit100 includes a priority valve 110, a steering and brake pressurecompensation valve 120, and a load sense cutoff valve 130. The dualpressure margin priority circuit 100 controls flow from a pressurecompensated hydraulic source or pump 140 to steering and brake valveinlets 150 and to low priority function inlets 160. The pressurecompensated hydraulic pump 140 includes a load sense input 142 and anoutlet port 144. The dual pressure margin priority circuit 100 is alsoconnected to a steering and brake valve dynamic load sense circuit 152and to low priority function load sense circuit 162.

In the embodiment of FIG. 1, the priority valve 110 and the steeringpressure compensation valve 120 are each 2-way proportional flow spoolvalves. The priority valve 110 includes an inlet port 112 and an outletport 114 where the inlet port 112 is connected to the outlet port 144 ofthe pump 140, and the outlet port 114 of the priority valve 110 isconnected to the low priority function inlets 160. The steering pressurecompensation valve 120 includes an inlet port 122 and an outlet port 124where the inlet port 122 is connected to the outlet port 144 of the pump140, and the outlet port 124 of the steering pressure compensation valve120 is connected to the steering valve inlets 150.

The steering pressure compensation valve 120 controls flow from the pump140 to the steering pressure valve inlets 150. The steering pressurecompensation valve 120 includes a spring 126 that biases the steeringpressure compensation valve 120 to allow flow from the pump 140 to thesteering valve inlets 150. The steering pressure compensation valve 120also includes a contributing load sense input 128 that aids the biasingforce of the spring 126, and an opposing load sense input 129 thatopposes the biasing force of the spring 126. The outlet 124 of thesteering pressure compensation valve 120 is also connected to anopposing load sense line 174 and to a dynamic load sense feed orifice156 having an inlet port 154 and an outlet port 158. The inlet port 154of the dynamic load sense feed orifice 156 is connected to the outlet124 of the steering pressure compensation valve 120, and the outlet port158 of the dynamic load sense feed orifice 156 is connected to a loadsense line 172. The dynamic load sense feed orifice 156 controls theamount of oil that flows through the load sense line 172.

The contributing load sense input 128 of the steering pressurecompensation valve 120 is connected to the load sense line 172 whichcommunicates the pressure from the outlet port 158 of the dynamic loadsense feed orifice 156. The opposing load sense input 129 of thesteering pressure compensation valve 120 is connected to the oppositionload sense line 174 which communicates the pressure from the outlet port124 of the steering pressure compensation valve 120. The pressure in theload sense line 172 at the contributing load sense input 128 aids thebiasing force of the spring 126, and the pressure in the opposition loadsense line 174 at the opposing load sense input 129 opposes thesebiasing forces. Thus, the contributing bias forces of the steeringpressure compensation valve 120 are the sum of the bias forces from thespring 126 and the contributing load sense input 128. The load senseline 172 also connects the outlet port of the dynamic load sense feedorifice 156 to the steering valve dynamic load sense circuit 152.

The connection of the load sense line 172 to the steering and brakevalve dynamic load sense circuit 152 can be through a dynamic load senseboost orifice 194 having an inlet port 192 and an outlet port 196. Whenthe boost orifice 194 is included, the inlet port 192 of the boostorifice 194 is connected to the outlet port 158 of the dynamic loadsense feed orifice 156 through load sense line 172, and the outlet port196 of the boost orifice 194 is connected to the steering and brakevalve dynamic load sense circuit 152.

The dual pressure margin priority circuit can include a load senseresolution shuttle valve 102 with a first inlet 104, a second inlet 106and an outlet 108. A load sense line 180 connects the steering valveload sense circuit 152 to the first inlet 104 of the shuttle valve 102.A load sense line 182 connects the low priority function load sensecircuit 162 to the second inlet 106 of the shuttle valve 102. The outlet108 of the shuttle valve 102 is connected to the load sense input 142 ofthe pump 140 such that the pump 140 is always responsive to the higherof the load pressure signals from the load sense line 180 (for thesteering valve load sense 152) and the load sense line 182 (for the lowpriority function load sense 162).

In the embodiment of FIG. 1, the load sense cutoff valve 130 is a 3-way,2-position spool valve. The load sense cutoff valve 130 includes a firstinlet port 132, a second inlet port 134, an outlet port 136, a loadsense input 137 and a spring 138. The spring 138 biases the load sensecutoff valve 130 to connect the first inlet port 132 to the outlet port136. The first inlet port 132 is connected to the load sense line 180and the second inlet port 134 is connected to a reservoir or tank 148.The outlet port 136 is connected to a load sense line 184. The loadsense line 180 is also connected to the load sense input 137 of the loadsense cutoff valve 130 such that the pressure of the load sense line 180at the load sense input 137 opposes the biasing force of the spring 138.When the biasing force of the spring 138 is greater than the pressure ofthe load sense line 180 at the load sense input 137, the load sensecutoff valve 130 connects the pressure from the load sense line 180 atthe first inlet port 132 to the load sense line 184 at the outlet port136. When the biasing pressure of the load sense line 180 at the loadsense input 137 is greater than the biasing force of the spring 138, theload sense cutoff valve 130 connects the tank 148 at the second inletport 134 to the load sense line 184 at the outlet port 136.

The priority valve 110 controls flow from the pump 140 to the lowpriority function inlets 160. The priority valve 110 includes a spring116 that biases the priority valve 110 to block flow from the pump 140to the low priority function inlets 160. The priority valve 110 alsoincludes a contributing load sense input 118 that aids the biasing forceof the spring 116, and an opposing load sense input 119 that opposes thebiasing force of the spring 116. The contributing load sense input 118is connected to the load sense line 184 from the outlet port 136 of theload sense cutoff valve 130. The opposing load sense input 119 isconnected to the opposition load sense line 174 connected to the outletport 124 of the steering pressure compensation valve 120. Thus, thecontributing bias forces of the priority valve 110 are the sum of thebias forces from the spring 116 and the contributing load sense input118. The margin between the contributing and opposing bias forces of thepriority valve 110 is less than the margin between the contributing andopposing bias forces of the steering pressure compensation valve 120.

The biasing force of the spring 138 of the load sense cutoff valve 130sets a cutoff pressure threshold, for example 175 bar. When the steeringload sense pressure communicated on load sense line 180 to the loadsense input 137 is less than the cutoff pressure threshold then the loadsense cutoff valve 130 communicates the steering load sense pressure onthe load sense line 180 to the outlet port 136 which is communicated tothe contributing load sense input 118 to aid the bias spring 116 of thepriority valve 110. In this condition, the contributing and opposingbias forces on the opposite sides of the spools of the priority valve110 and the steering pressure compensation valve 120 are approximatelythe same except for the difference between the biasing force of thespring 116 of the priority valve 110 and the biasing force of the spring126 of the steering pressure compensation valve 120. As noted above, themargin between the contributing and opposing bias forces of the priorityvalve 110 is less than the margin between the contributing and opposingbias forces of the steering pressure compensation valve 120. When thesteering load sense pressure communicated on load sense line 180 to theload sense input 137 is less than the cutoff pressure threshold, thepriority valve 110 controls the allocation of flow from the pump 140 tothe steering valve inlets 150 and to the low priority function inlets160, and the priority valve 110 controls the steering valve dynamic loadsense 152. When the steering load sense pressure communicated on loadsense line 180 to the load sense input 137 is less than the cutoffpressure threshold and there is no fluid flow to the low priorityfunction inlets 160, the steering and brake pressure compensation valve120 controls the allocation of flow from the pump 140 to the steeringvalve inlets 150 and controls the steering valve dynamic load sense 152.

There is a pressure margin or difference between the pressure at theoutput 144 of the pump 140 and the load sense pressure in load senseline 180, for example if the maximum pressure of pump 140 is 200 bar andwhen the pump 140 is at maximum pressure the pressure in line 180 couldbe 175 bar defining a pump pressure margin of 25 bar. The cutoffpressure threshold of the load sense cutoff valve 130 can be set toapproximately equal the pressure value in load sense line 180 when thepump 140 is at maximum pressure, which in this example would be 175 bar.When the steering load sense pressure communicated on load sense line180 to the load sense input 137 is greater than the cutoff pressurethreshold, the load sense cutoff valve 130 connects the load sense line184 to tank 148 which disables the priority valve 110. With essentiallyno assistance from pressure at the contributing load sense input 118,the pressure in opposition load sense line 174 at the opposing loadsense input 119 overcomes the bias force of the spring 116 and fullyopens the priority valve 110. The pressure on load sense line 172 at thecontributing load sense input 128 along with the bias force of spring126 overcomes the opposing pressure of the opposition load sense line174 at the opposing load sense input 129 to also fully open the steeringpressure compensation valve 120.

Thus when steering and brake load sense pressure on load sense line 180at the load sense input 137 is greater than the cutoff pressurethreshold, full pressure from the pump 140 is available to the steeringand brake valve inlets 150 as long as the demanded pump flow from thesteering and brake inlets 150 and low priority function inlets 160 doesnot exceed the maximum flow capacity of the pump 140. In the case wherepump flow capacity is exceeded, then flow pressure equal to the pumpcontroller pressure margin plus the cutoff pressure threshold isavailable at the steering and brake valve inlets 150.

A typical load sense relief circuit limits pressure to the steeringvalve inlets 150 to the maximum pump pressure minus the pump controllerpressure margin, which in our example is 175 bar, to prevent blockingflow to the low priority function inlets 160. The dual pressure marginpriority circuit 100 opens flow to both the low priority function inlets160 and the steering and brake valve inlets 150 when the steering andbrake load sense pressure is greater than or equal to the cutoffpressure threshold, so that substantially all of the maximum pumppressure, in this example 200 bar, is available to both the steering andbrake valve inlets 150 and the low priority function inlets 160. Thus,this increases the available pressure by the pump controller pressuremargin, in this case 25 bar. In this configuration, priority ismaintained for the steering valve inlets 150 until the pump 140 reachesmaximum pressure. When the pump 140 is at or near maximum pressure, boththe priority valve 110 and the steering pressure compensation valve 120are fully open to maximize the pressure at both the steering valveinlets 150 and the low priority function inlets 160 and to provideapproximately equal pressure to both the steering valve inlets 150 andthe low priority function inlets 160.

FIG. 2 illustrates another exemplary embodiment of a dual pressuremargin priority circuit 200 that can be used to enable the steeringcircuit to utilize greater pump pressure. The dual pressure marginpriority circuit 200 also includes a priority valve 110, a steering andbrake pressure compensation valve 120, and a load sense cutoff valve130, where the dual pressure margin priority circuit 200 controls flowfrom a pressure compensated hydraulic source or pump 140 to steering andbrake valve inlets 150 and to low priority function inlets 160. Theprimary difference in the dual pressure margin priority circuit 200 ofFIG. 2 is that the first inlet port 132 and the load sense input 137 ofthe load sense cutoff valve 130 are connected to the load sense line 172instead of the load sense line 180. Both of the load sense lines 172 and180 are connected to the steering and brake dynamic load sense inputs152. However, if a dynamic load sense boost orifice 194 is included,then the load sense line 172 is connected to the inlet port 192 of theboost orifice 194, and the load sense line 180 and the steering dynamicload sense inputs 152 are connected to the outlet port 196 of the boostorifice 194. In the dual pressure margin priority circuit 200, the boostorifice 194 will not affect load sense pressure between the outlet port158 of the dynamic load sense feed orifice 156 and the load sense input137 of the load sense cutoff valve 130 which is connected directly tothe load sense line 172. Whereas in the dual pressure margin prioritycircuit 100 of FIG. 1, the boost orifice 194 will affect load sensepressure between the outlet port 158 of the dynamic load sense feedorifice 156 and the load sense input 137 of the load sense cutoff valve130 which is connected directly to the load sense line 180. The dualpressure margin priority circuit 200 of FIG. 2 can provide moreconsistent pre-boost steering and brake dynamic load sense flow for someapplications.

FIG. 3 illustrates yet another exemplary embodiment of a dual pressuremargin priority circuit 300 that can be used to enable the steeringcircuit to utilize greater pump pressure. The dual pressure marginpriority circuit 300 also includes a priority valve 110 and a steeringand brake pressure compensation valve 120, but it uses a pressurelimiter valve 330 as the load sense cutoff valve. As in the priorembodiments, the dual pressure margin priority circuit 300 controls flowfrom a pressure compensated hydraulic source or pump 140 to steering andbrake valve inlets 150 and to low priority function inlets 160.

The pressure limiter load sense cutoff valve 330 is a 2-way, 2-positionspool valve that includes an inlet port 332, an outlet port 336, a loadsense input 337 and a spring 338. The inlet port 332 is connected to theload sense line 180 and the outlet port 336 is connected to a reservoiror tank 148. The spring 338 biases the load sense cutoff valve 330 toblock the inlet port 332 and the outlet port 336. The load sense line180 is also connected to the load sense input 337 of the pressurelimiter valve 330 such that the pressure of the load sense line 180 atthe load sense input 337 opposes the biasing force of the spring 338.When the biasing force of the spring 338 is greater than the pressure ofthe load sense line 180 at the load sense input 337, the pressurelimiter valve 330 blocks the inlet port 332 and the outlet port 336.When the biasing pressure of the load sense line 180 at the load senseinput 337 is greater than the biasing force of the spring 338, thepressure limiter valve 330 connects the inlet port 332 to the outletport 336 which connects the load sense line 180 to the tank 148. Thus,the pressure limiter valve 330 limits the pressure in the load senseline 180 to the level that balances the biasing force of the spring 338.

The priority valve 110 controls flow from the pump 140 to the lowpriority function inlets 160. The priority valve 110, as above, includesa spring 116 that biases the priority valve 110 to block flow from thepump 140 to the low priority function inlets 160. The priority valve 110also includes a contributing load sense input 118 that aids the biasingforce of the spring 116, and an opposing load sense input 119 thatopposes the biasing force of the spring 116. However, in the embodimentof FIG. 3, the contributing load sense input 118 is connected to a loadsense line 384 that is directly connected to the load sense line 180.The opposing load sense input 119 is connected to the opposition loadsense line 174 connected to the outlet port 124 of the steering pressurecompensation valve 120. Thus, the contributing bias forces of thepriority valve 110 are the sum of the bias forces from the spring 116and the contributing load sense input 118 which is connected to the loadsense line 384. The margin between the opposing bias forces of thepriority valve 110 is less than the margin between the opposing biasforces of the steering pressure compensation valve 120.

The biasing force of the spring 338 of the pressure limiter valve 330sets a cutoff pressure threshold, for example 175 bar. When the steeringload sense pressure communicated on load sense line 180 to the loadsense input 337 is less than the cutoff pressure threshold then thepressure limiter valve 330 blocks the inlet port 332 and the steeringload sense pressure on the load sense line 180 is communicated to thecontributing load sense input 118 to aid the bias spring 116 of thepriority valve 110. In this condition, the contributing and opposingbias forces on the opposite sides of the spools of the priority valve110 and the steering pressure compensation valve 120 are approximatelythe same except for the difference between the biasing force of thespring 116 of the priority valve 110 and the biasing force of the spring126 of the steering pressure compensation valve 120. As noted above, themargin between the contributing and opposing bias forces of the priorityvalve 110 is less than the margin between the contributing and opposingbias forces of the steering pressure compensation valve 120. When thesteering load sense pressure communicated on load sense line 180 to theload sense input 337 is less than the cutoff pressure threshold, thepriority valve 110 controls the allocation of flow from the pump 140 tothe steering valve inlets 150 and to the low priority function inlets160, and the priority valve 110 controls the steering valve dynamic loadsense 152. When the steering load sense pressure communicated on loadsense line 180 to the load sense input 337 is less than the cutoffpressure threshold and there is no fluid flow to the low priorityfunction inlets 160, the steering and brake pressure compensation valve120 controls the allocation of flow from the pump 140 to the steeringvalve inlets 150 and controls the steering valve dynamic load sense 152.

There is a pressure margin or difference between the pressure at theoutput 144 of the pump 140 and the load sense pressure in load senseline 180, for example if the maximum pressure of pump 140 is 200 bar andwhen the pump 140 is at maximum pressure the pressure in line 180 couldbe 175 bar defining a pump pressure margin of 25 bar. The cutoffpressure threshold of the load sense cutoff valve 330 can be adjusted toapproximately equal the pressure value in load sense line 180 when thepump 140 is at maximum pressure, which in this example would be 175 bar.The pressure limiter valve 330 limits the steering load sense pressurecommunicated on load sense line 180 to the cutoff pressure threshold setby the bias spring 338. The pressure limiter valve 330 releases anyexcess pressure of the steering valve dynamic load sense lines includingon the load sense lines 180 and 384 to tank 148. This limited conditioncommunicates the threshold pressure on load sense line 384 to thecontributing load sense input 118 to aid the bias force of spring 116opposing the pressure in opposing load sense line 174 communicated tothe opposing load sense input 119. This fully opens the priority valve110. The pressure on load sense line 172 at the contributing load senseinput 128 along with the bias force of spring 126 overcomes the opposingpressure of the opposition load sense line 174 at the opposing loadsense input 129 to also fully open the steering pressure compensationvalve 120.

The dual pressure margin priority circuit 300 of FIG. 3 can provide alower cost option because the 2-way, 2-position pressure limiter spoolvalve 330 is lower cost than the 3-way, 2-position selector spool valve130. The pressure limiter spool valve 330 can be manufactured with asimple spool or poppet and a spring. The load sense cutoff selectorspool valve 130 requires a more complex spool (3-way vs 2-way) and moreinternal manifold porting. The trade-off is part cost for oil flow. Thepressure limiter does waste some oil when in the pressure limiting mode.

The primary difference in the dual pressure margin priority circuit 400of FIG. 4 and the dual pressure margin priority circuit 300 of FIG. 3 isthat the first inlet port 332 and the load sense input 337 of thepressure limiter valve 330, and the load sense line 384 are connected tothe load sense line 172 instead of the load sense line 180. Both of theload sense lines 172 and 180 are connected to the steering and brakedynamic load sense inputs 152. However, if a dynamic load sense boostorifice 194 is included, then the load sense line 172 is connected tothe inlet port 192 of the boost orifice 194, and the load sense line 180and the steering dynamic load sense inputs 152 are connected to theoutlet port 196 of the boost orifice 194. In the dual pressure marginpriority circuit 400 the boost orifice 194 will not affect load sensepressure between the outlet port 158 of the dynamic load sense feedorifice 156 and the load sense input 337 of the pressure limiter valve330 which is connected directly to the load sense line 172. Whereas inthe dual pressure margin priority circuit 300 of FIG. 3, the boostorifice 194 will affect load sense pressure between the outlet port 158of the dynamic load sense feed orifice 156 and the load sense input 337of the pressure limiter valve 330 which is connected directly to theload sense line 180. The dual pressure margin priority circuit 400 ofFIG. 4 can provide more consistent pre-boost steering and brake dynamicload sense flow for some applications.

While the disclosure has been illustrated and described in detail in thedrawings and foregoing description, such illustration and description isto be considered as exemplary and not restrictive in character, it beingunderstood that illustrative embodiment(s) have been shown and describedand that all changes and modifications that come within the spirit ofthe disclosure are desired to be protected. It will be noted thatalternative embodiments of the present disclosure may not include all ofthe features described yet still benefit from at least some of theadvantages of such features. Those of ordinary skill in the art mayreadily devise their own implementations that incorporate one or more ofthe features of the present disclosure and fall within the spirit andscope of the present invention as defined by the appended claims.

I claim:
 1. A dual pressure margin priority circuit that controls flowfrom a hydraulic pump to steering valve inlets and to low priorityfunction inlets, the dual pressure margin priority circuit comprising: asteering pressure compensation valve with an inlet port and an outletport, the inlet port hydraulically coupled to the pump and the outletport hydraulically coupled to the steering valve inlets; a load sensefeed orifice with an inlet port and an outlet port, the inlet port ofthe load sense feed orifice hydraulically coupled to the outlet port ofthe steering pressure compensation valve to provide a steering valvedynamic load sense pressure; a load sense cutoff valve with a firstinlet port and an outlet port, the first inlet port of the load sensecutoff valve hydraulically coupled to the outlet port of the load sensefeed orifice; a priority valve with an inlet port and an outlet port,the inlet port of the priority valve hydraulically coupled to the pumpand the outlet port of the priority valve hydraulically coupled to thelow priority function inlets; wherein the load sense cutoff valvecontrols flow from the pump to the low priority function inlets throughthe priority valve based on the steering valve dynamic load sensepressure at the first inlet port of the load sense cutoff valve.
 2. Thedual pressure margin priority circuit of claim 1, wherein the pump is apressure compensated hydraulic pump that includes a load sense input,and the dual pressure margin priority circuit further comprises: a loadsense resolution shuttle valve with a first inlet port, a second inletport and an outlet port, the first inlet port hydraulically coupled tothe steering valve dynamic load sense pressure, the second inlet porthydraulically coupled to a low priority function load sense pressure andthe outlet port hydraulically coupled to the load sense input of thepump; wherein the load sense resolution shuttle valve communicates thegreater of the steering valve dynamic load sense pressure at the firstinlet port and the low priority function load sense pressure at thesecond inlet port to the load sense input of the pump.
 3. The dualpressure margin priority circuit of claim 2, wherein the load sensecutoff valve further includes a second inlet port, a load sense inputand a bias spring, the second inlet port of the load sense cutoff valvehydraulically coupled to a tank, and the load sense input of the loadsense cutoff valve hydraulically coupled to the steering valve dynamicload sense pressure at the first inlet port of the load sense cutoffvalve; wherein when the force of the bias spring is greater than theforce of the steering valve dynamic load sense pressure at the loadsense input of the load sense cutoff valve, the load sense cutoff valvecommunicates the steering valve dynamic load sense pressure at the firstinlet port to the outlet port; and when the force of the bias spring isless than the force of the steering valve dynamic load sense pressure atthe load sense input of the load sense cutoff valve, the load sensecutoff valve connects the outlet port to the tank.
 4. The dual pressuremargin priority circuit of claim 3, wherein the force of the bias springof the load sense cutoff valve is adjustable.
 5. The dual pressuremargin priority circuit of claim 3, wherein the priority valve is a2-way proportional flow spool valve that further includes a bias spring,a contributing load sense input and an opposing load sense input, thecontributing load sense input of the priority valve hydraulicallycoupled to the outlet port of the load sense cutoff valve and theopposing load sense input of the priority valve hydraulically coupled tothe outlet port of the steering pressure compensation valve; whereinpressure at the contributing load sense input of the priority valve aidsthe biasing force of the bias spring to stop flow from the inlet port tothe outlet port of the priority valve, and pressure at the opposing loadsense input of the priority valve opposes the biasing force of the biasspring to open flow from the inlet port to the outlet port of thepriority valve.
 6. The dual pressure margin priority circuit of claim 5,wherein the steering pressure compensation valve is a 2-way proportionalflow spool valve that further includes a bias spring, a contributingload sense input and an opposing load sense input, the contributing loadsense input of the steering pressure compensation valve hydraulicallycoupled to the outlet port of the load sense feed orifice and theopposing load sense input of the steering pressure compensation valvehydraulically coupled to the outlet port of the steering pressurecompensation valve; wherein pressure at the contributing load senseinput of the steering pressure compensation valve aids the biasing forceof the bias spring to open flow from the inlet port to the outlet portof the steering pressure compensation valve, and pressure at theopposing load sense input of the steering pressure compensation valveopposes the biasing force of the bias spring to stop flow from the inletport to the outlet port of the steering pressure compensation valve. 7.The dual pressure margin priority circuit of claim 6, further comprisinga load sense boost orifice with an inlet port and an outlet port, theinlet port of the load sense boost orifice hydraulically coupled to theoutlet port of the load sense feed orifice, the outlet port of the loadsense boost orifice hydraulically coupled to the first inlet port of theload sense cutoff valve and to the first inlet port of the load senseresolution shuttle valve.
 8. The dual pressure margin priority circuitof claim 6, further comprising a load sense boost orifice with an inletport and an outlet port, the inlet port of the load sense boost orificehydraulically coupled to the outlet port of the load sense feed orifice,the outlet port of the load sense boost orifice hydraulically coupled tothe first inlet port of the load sense resolution shuttle valve.
 9. Thedual pressure margin priority circuit of claim 2, wherein the load sensecutoff valve further includes a load sense input and a bias spring, theload sense input of the load sense cutoff valve hydraulically coupled tothe steering valve dynamic load sense pressure at the first inlet portof the load sense cutoff valve and the outlet port of the load sensecutoff valve hydraulically coupled to a tank; wherein when the force ofthe bias spring is greater than the force of the steering valve dynamicload sense pressure at the load sense input of the load sense cutoffvalve, the load sense cutoff valve blocks flow from the inlet port tothe outlet port of the load sense cutoff valve; and when the force ofthe bias spring is less than the force of the steering valve dynamicload sense pressure at the load sense input of the load sense cutoffvalve, the load sense cutoff valve opens flow from the inlet port to theoutlet port of the load sense cutoff valve.
 10. The dual pressure marginpriority circuit of claim 9, wherein the force of the bias spring of theload sense cutoff valve is adjustable.
 11. The dual pressure marginpriority circuit of claim 9, wherein the priority valve is a 2-wayproportional flow spool valve that further includes a bias spring, acontributing load sense input and an opposing load sense input, thecontributing load sense input of the priority valve hydraulicallycoupled to the inlet port of the load sense cutoff valve and theopposing load sense input of the priority valve hydraulically coupled tothe outlet port of the steering pressure compensation valve; whereinpressure at the contributing load sense input of the priority valve aidsthe biasing force of the bias spring to stop flow from the inlet port tothe outlet port of the priority valve, and pressure at the opposing loadsense input of the priority valve opposes the biasing force of the biasspring to open flow from the inlet port to the outlet port of thepriority valve.
 12. The dual pressure margin priority circuit of claim11, wherein the steering pressure compensation valve is a 2-wayproportional flow spool valve that further includes a bias spring, acontributing load sense input and an opposing load sense input, thecontributing load sense input of the steering pressure compensationvalve hydraulically coupled to the outlet port of the load sense feedorifice and the opposing load sense input of the steering pressurecompensation valve hydraulically coupled to the outlet port of thesteering pressure compensation valve; wherein pressure at thecontributing load sense input of the steering pressure compensationvalve aids the biasing force of the bias spring to open flow from theinlet port to the outlet port of the steering pressure compensationvalve, and pressure at the opposing load sense input of the steeringpressure compensation valve opposes the biasing force of the bias springto stop flow from the inlet port to the outlet port of the steeringpressure compensation valve.
 13. The dual pressure margin prioritycircuit of claim 12, further comprising a load sense boost orifice withan inlet port and an outlet port, the inlet port of the load sense boostorifice hydraulically coupled to the outlet port of the load sense feedorifice, the outlet port of the load sense boost orifice hydraulicallycoupled to the first inlet port of the load sense cutoff valve and tothe first inlet port of the load sense resolution shuttle valve.
 14. Thedual pressure margin priority circuit of claim 12, further comprising aload sense boost orifice with an inlet port and an outlet port, theinlet port of the load sense boost orifice hydraulically coupled to theoutlet port of the load sense feed orifice, the outlet port of the loadsense boost orifice hydraulically coupled to the first inlet port of theload sense resolution shuttle valve.
 15. A method of controlling flowfrom a hydraulic pump to steering valve inlets and to low priorityfunction inlets, the method comprising: controlling flow from the pumpto the steering valve inlets using a steering valve; sensing a steeringvalve dynamic load sense pressure at an outlet port of the steeringvalve; controlling flow from the pump to the low priority functioninlets using a priority valve, the priority valve being separate fromthe steering valve; controlling a load sense cutoff valve using thesteering valve dynamic load sense pressure; controlling flow from thepump to the low priority function inlets through the priority valveusing the load sense cutoff valve based on the steering valve dynamicload sense pressure.
 16. The method of claim 15, wherein controlling aload sense cutoff valve using the steering valve dynamic load sensepressure comprises: hydraulically coupling the steering valve dynamicload sense pressure through the load sense cutoff valve from an inletport of the load sense cutoff valve to a load sense input of thepriority valve when the steering valve dynamic load sense pressure isless than a threshold pressure; and hydraulically coupling the loadsense input of the priority valve through the load sense cutoff valvefrom an outlet port of the load sense cutoff valve to a tank when thesteering valve dynamic load sense pressure is greater than a thresholdpressure.
 17. The method of claim 16, wherein controlling flow from thepump to the low priority function inlets using a priority valvecomprises: hydraulically coupling the outlet port of the load sensecutoff valve to a contributing load sense input of the priority valve;hydraulically coupling the outlet port of the steering valve to anopposing load sense input of the priority valve; and biasing thepriority valve to block flow from the pump to the low priority functioninlets using a priority bias spring force and the contributing loadsense input of the priority valve; biasing the priority valve to openflow from the pump to the low priority function inlets using theopposing load sense input of the priority valve.
 18. The method of claim17, wherein controlling flow from the pump to the steering valve inletsusing a steering valve comprises: hydraulically coupling the outlet portof the steering valve to an inlet port of a load sense feed orifice;hydraulically coupling an outlet port of the load sense feed orifice toa contributing load sense input of the steering valve; hydraulicallycoupling the outlet port of the steering valve to an opposing load senseinput of the steering valve; and biasing the steering valve to open flowfrom the pump to the steering valve inlets using a steering bias springforce and the contributing load sense input of the steering valve;biasing the priority valve to block flow from the pump to the steeringvalve inlets using the opposing load sense input of the steering valve.19. The method of claim 15, wherein controlling flow from the pump tothe low priority function inlets using a priority valve and controllinga load sense cutoff valve using the steering valve dynamic load sensepressure comprises: hydraulically coupling the steering valve dynamicload sense pressure to an inlet port of the load sense cutoff valve andto a contributing load sense input of the priority valve; blocking thesteering valve dynamic load sense pressure at the inlet port of the loadsense cutoff valve when the steering valve dynamic load sense pressureis less than a threshold pressure; and hydraulically coupling the inletport of the load sense cutoff valve and the contributing load senseinput of the priority valve through the load sense cutoff valve to atank when the steering valve dynamic load sense pressure is greater thana threshold pressure; hydraulically coupling the outlet port of thesteering valve to an opposing load sense input of the priority valve;biasing the priority valve to block flow from the pump to the lowpriority function inlets using a priority bias spring force and thecontributing load sense input of the priority valve; and biasing thepriority valve to open flow from the pump to the low priority functioninlets using the opposing load sense input of the priority valve. 20.The method of claim 19, wherein controlling flow from the pump to thesteering valve inlets using a steering valve comprises: hydraulicallycoupling the outlet port of the steering valve to an inlet port of aload sense feed orifice; hydraulically coupling an outlet port of theload sense feed orifice to a contributing load sense input of thesteering valve; hydraulically coupling the outlet port of the steeringvalve to an opposing load sense input of the steering valve; and biasingthe steering valve to open flow from the pump to the steering valveinlets using a steering bias spring force and the contributing loadsense input of the steering valve; biasing the priority valve to blockflow from the pump to the steering valve inlets using the opposing loadsense input of the steering valve.